#173 When faac -w, usage of -o - should be warned to user

When faac is used with -w (wrap into MP4 container), "-o -" option means outputting to normal file in the current directory (./-). This behavior should be documented and/or warned to the standard error. (Also, I think just aborting the process is OK). I think showing that whether stdin/stdout is being used in diagnostic message is always good.